1. Field of the Invention
This invention relates to a light-emitting device for generating light of a predetermined color such as light of a white color by use of a plurality of light-emitting elements (LED: Light Emitting Diode), particularly three LED of a red color (R), a green color (G) and a blue color (B).
2. Description of the Related Art
A light source using a combination of the three primary colors of a red color (hereinafter called “R”), a green color (hereinafter called “G”) and a blue color (hereinafter called “B”) has been utilized as a white color light source generated by synthesizing the three primary colors. An LED for realizing a pseudo-white color by combining blue color light and a fluorescent substance excited by blue color light and generating red color light is also known.
The white color light source has been used as a backlight of a liquid crystal display device (LCD) of a cellular telephone unit, or the like.
To emit white color light by use of the LED of the three primary colors, it is necessary to appropriately select the arrangement of the LED of the three primary colors and light intensity and to appropriately adjust the color balance of the three primary colors for avoiding color shift. Various proposals have been made as far as to the arrangement of the three LED and the light-emitting methods (refer to patent references 1 and 2).
Patent Reference 1: Japanese Unexamined Utility Model Publication No. Hei 6(1994)-79165
Patent Reference 2: Japanese Unexamined Utility Model Publication No. Hei 5(1993)-21458
Patent reference 1 discloses an LED lamp having a construction in which one red LED is arranged at a center and two green LED and two blue LED are so arranged as to be mutually symmetric for each display color with the red LED being the center.
Patent reference 2 discloses a semiconductor light-emitting device having a construction in which one red LED is disposed at a center of a square and two green LED and two blue LED are alternately disposed at the four corners of the square.
When LED of the three primary colors are used to create a light source of a desired color, the color of each LED, the color balance and light intensity are important factors to keep display color quality of the desired color and to prevent color shift. Particularly, a light intensity ratio of each color determines the tone of the color.
It is difficult to completely fabricate the LED chip of each of R, G and B as designed. When a plurality of LED of the same color is used or when a large number of different LED are used to create a light emission color of the same color tone, deviation of the color balance occurs from product to product.
In the case of a white color, in particular, it is of importance to create the color tone as designed because the atmosphere of a room under a daylight color, a lamp color, etc, changes when the color tone changes.
A degradation speed of each LED with the passage of time is different. Due to this difference, differences also occur in the color and the color balance in the course of use of the light-emitting device for a long time and the color tone of the light emission color changes.
To cope with this change with time, it is believed that it is desirable to dispose a light intensity monitor chip for each LED, to adjust the light emission intensity of each LED while the light intensity of each LED is always measured, and thus to keep quality of the light emission color. When the light intensity monitor chip is individually disposed for each LED, however, the number of components for monitoring the light intensity increases, the circuit becomes more complicated and the size of a package becomes greater. Therefore, this construction is not suitable for the application for which a compact size is particularly requisite.